Display device and method for manufacturing the same

ABSTRACT

An object of the present invention is to decrease substantial resistance of an electrode such as a transparent electrode or a wiring, and furthermore, to provide a display device for which is possible to apply same voltage to light-emitting elements. In the invention, a auxiliary wiring that is formed in one layer in which a conductive film of a semiconductor element such as an electrode, wiring, a signal line, a scanning line, or a power supply line is connected to an electrode typified by a second electrode, and a wiring. It is preferable that the auxiliary wiring is formed into a conductive film to include low resistive material, especially, formed to include lower resistive material than the resistance of an electrode and a wiring that is required to reduce the resistance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device equipped with alight-emitting element and a method for manufacturing the same.

2. Description of the Related Art

In recent years, a large-sized screen and high-definition are promotedin a display device having a light-emitting element and a liquid crystalelement, which the number of wirings such as a signal line and ascanning line, and the length of a wiring tend to increase. Therefore,it is necessary to prevent voltage drop due to wiring resistance, asignal writing defect, a gradation defect, and the like.

Thus, there is a configuration in which an auxiliary wiring made of atransparent conductive film is connected to the transparent electrodethat the light-emitting element has, interposing an anisotropicalconductor (refer to Patent Document 1). According to Patent Document 1,effective resistance of the transparent electrode can be lowered.Furthermore, it is possible to apply constant voltage to thelight-emitting element; therefore, it is mentioned that a display defectsuch as display unevenness can be prevented. [Patent Document 1]Japanese Patent Laid-Open No. 2002-33198

According to a method different from Patent Document 1, an object of theinvention is to provide a display device that can reduce effectiveresistance of an electrode such as a transparent electrode and a wiring,and furthermore which can apply constant voltage to a light-emittingelement.

The display device includes a light-emitting element having a firstelectrode and a second electrode to apply voltage to a light-emittinglayer. The second electrode can be shared in light-emitting elements,that is the second electrode can be formed without patterning over thelight-emitting layer in pixels. It is necessary for such secondelectrode to apply same voltage to the light-emitting elements.

In addition, when light from the light-emitting layer is emitted to anopposite side of a substrate in which a semiconductor element typifiedby a TFT is provided (hereinafter, referred to as a top emission), thesecond electrode needs to be transparent. Therefore, the secondelectrode has a configuration having a transparent conductive film, forexample, an ITO (indium tin oxide). However, the resistance of thetransparent conductive film is high. Furthermore, the second electrodemay use a thin film of a metal film; however, the resistance has becomehigh due to the thin film-thickness. As a result, it is concerned thatlow power consumption of the display device is disturbed.

Especially, as a display device gets larger in size, it becomes moreimportant to apply constant voltage to the light-emitting layer.However, as mentioned above, resistance of the second electrode is high,and consequently it is concerned that power consumption of a displaydevice is increased.

SUMMARY OF THE INVENTION

Thus, an object of the present invention is to provide a display devicewhich reduces substantial resistance of a second electrode, and whichhave a new configuration that can apply constant voltage to alight-emitting element.

In the above problems, one feature of the invention is that a conductivefilm (hereinafter, referred to as an auxiliary wiring) is connected toan electrode typified by the above second electrode, and a wiring.

It is preferable that the auxiliary wiring is formed in a conductivefilm to include low resistive material, especially, formed to includelower resistive material than the resistance of an electrode and awiring that needs to reduce the resistance. Specifically, it can beformed to include an element selected from the group consisting Ta, W,Ti, Mo, Al, and Cu, an alloy material or a compound material mainlycontaining the element, or a transparent conductive film such as ITO andSnO₂. In addition, even the case of using an ITO whose height of theresistance is concerned as the auxiliary wiring, the auxiliary wiring isprovided, so that the substantial resistance of the second electrode canbe reduced.

The above auxiliary wiring can be formed by sputtering, plasma CVD,vapor deposition, printing, or spin coating. The auxiliary wiring may beassumed to have a predetermined shape by using a mask, and furthermore,may have a predetermined shape by etching such as dry etching or wetetching.

Especially, the invention is different from Reference 1 in which theauxiliary wiring connected to a transparent electrode is newly formed.In the invention, the auxiliary wiring is formed in one layer in which aconductive film of a semiconductor element such as an electrode and awiring, a signal line, a scanning line, or a power supply line isformed. Furthermore, the auxiliary wiring is formed over an insulatingfilm in which a conductive film of a semiconductor element such as anelectrode and a wiring, a signal line, a scanning: line, or a powersupply line is formed. More preferably, the auxiliary wiring is formedby using the same material as the conductive film for an electrode and awiring, a signal line, a scanning line, or a power supply line of asemiconductor element. Consequently, it is not necessary to provide astep of forming the auxiliary wiring, thereby not increasing the maskfor the auxiliary wiring.

As the semiconductor element, a thin film transistor (TFT) using anon-single crystal semiconductor film typified by amorphous silicon andpolycrystalline silicon, a MOS transistor formed using a semiconductorsubstrate and a SOI substrate, a junction transistor, a transistor withthe use of an organic semiconductor and carbon nanotube, and othertransistors can be applied.

For example, when using a TFT as a semiconductor element, a firstinsulating film provided by covering at least a gate electrode providedover a semiconductor film, and a second insulating film provided overthe first insulating film are included. As the insulating films arelaminated, an area for providing the auxiliary wiring can be enlarged,which can decrease the resistance much more.

The first insulating film and the second insulating film can be formedfrom an inorganic material containing silicon such as silicon oxide,silicon nitride or silicon nitride oxide, or from an organic materialcontaining a material such as polyimide, polyamide, acryl, BCB(benzocyclobutene) or a resist. Furthermore, in order to realize aplanarization, the first insulating film, the second insulating film,and the like may be polished with a physical means such as CMP (ChemicalMechanical Polishing).

The auxiliary wiring may be used for a wiring to be lead (hereinafter,lead wiring) for connecting to an external circuit. The lead wiring isprovided along the circumference of a panel up to the connecting partwith the external circuit, which is preferable to be formed with theauxiliary wiring with a much lower resistance.

In the invention, the auxiliary wiring may be connected to a wiring thatlow resistance is required, which is not limited to a configuration inwhich the auxiliary wiring is connected to the second conductiveelectrode (transparent conductive film).

The invention is not limited to the provision of the auxiliary wiringfor the display device comprising a light-emitting element. Theauxiliary wiring may be provided also for a display layer comprising aliquid crystal element, and the resistance of an electrode and a wiringmay be reduced.

According to the auxiliary wiring of the invention, the substantialresistance of an electrode typified by the second electrode, and awiring can be reduced. The substantial resistance refers to combinedresistance of an electrode or a wiring. As a result, reduction in powerconsumption and prevention of voltage drop due to an electrode and awiring in the display device can be obtained.

In addition, a signal writing defect, a gradation defect, and the likedue to wiring resistance can be prevented. Furthermore, in the case ofthe second electrode, the generation of voltage drop can be controlledby connecting it to the auxiliary wiring, so that it is possible toapply uniform amount of voltage to the light-emitting element.Consequently, the improvement of the display quality can be obtained.

Especially in a large display device, an advantageous effect of reducingsubstantial resistance of an electrode or a wiring is remarkable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are views showing cross sections of a pixel portion of adisplay device of the present invention;

FIGS. 2A and 2B are views showing a display device of the invention;

FIG. 3 is a view showing an auxiliary wiring in a display device of theinvention;

FIG. 4 is a view showing an auxiliary wiring in a display device of theinvention;

FIG. 5 is a view showing an auxiliary wiring in a display device of theinvention;

FIG. 6 is a view showing an auxiliary wiring in a display device of theinvention;

FIGS. 7A to 7E are views showing pixel circuits of a display device ofthe invention;

FIG. 8 is a top view showing a pixel portion of a display device of theinvention;

FIGS. 9A to 9C are views showing cross sections of a pixel portion of adisplay device of the invention;

FIGS. 10A to 10C are views showing cross sections of a pixel portion ofa display device of the invention;

FIG. 11 is a graph showing a calculation result of the invention;

FIG. 12 is a graph showing a calculation result of the invention; and

FIGS. 13A to 13C are views showing electronic devices of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment mode of the present invention will be described below withreference to the accompanying drawings. Note that in all figures fordescribing the embodiment mode, the same reference numerals denote thesame parts or parts having the same function and the explanation willnot be repeated.

(Embodiment Mode 1)

In this embodiment mode, a configuration of a pixel portion of a displaydevice comprising an auxiliary wiring is described.

Configurations are shown in FIGS. 1A to 1C. That is, the configurationof a p-channel type TFT using polycrystalline silicon (polycrystallineTFT) as an example of a semiconductor element, and a pixel portion ofthe display device in which a transparent conductive film is employed asan example of the second electrode and in which the transparentconductive film that is a second electrode is connected to the auxiliarywiring.

In FIG. 1A, a configuration in which the second electrode is connectedto the auxiliary wiring, and in which the auxiliary wiring is formed inone layer in which a first electrode is formed is shown. Note that theauxiliary wiring may be formed of either the same material as the firstelectrode or a different material.

The pixel portion of the display device comprises a base insulating film11, a semiconductor film 12, a gate insulating film 14, a gate electrode15, a protective film 23, first to third insulating films 16 to 18, afirst electrode 19 for applying voltage to a light-emitting layer 20, alight-emitting layer 20, and a second electrode 21 formed sequentiallyover an insulating surface 10, and includes an auxiliary wiring 25 inone layer in which the first electrode 19 is formed.

An amorphous semiconductor film, for example, an amorphous silicon filmis formed over the base insulating film 11. The semiconductor film 12 ofan island shape is formed by patterning the amorphous silicon film intoa predetermined shape. The base insulating film 11 may have aconfiguration in which an insulating film including silicon islaminated, for example, a configuration in which an insulating film suchas a silicon oxide film, a silicon nitride film, or a silicon oxynitridefilm is laminated.

The semiconductor film 12 is crystallized with a laser or by heating.The gate electrode 15 is formed over the semiconductor film (crystallinesemiconductor film) that is crystallized. The gate electrode 15 may havea configuration in which a conductive film is laminated, for example, aconfiguration in which a TaN film is laminated over a W film. Animpurity region 13 is formed in a self-alignment manner by using thegate electrode 15 as a mask. For example, the first insulating film 16has an inorganic material, which is formed to cover the gate electrodeand the semiconductor film.

Thereafter, the protective film 23 is heated under the condition thatthe protective film 23 is formed to cover the gate electrode 15, whichthe semiconductor film may be recrystallized. Especially in the case offorming the protective film 23 with CVD, source gas may be controlled tocontain much hydrogen.

The wirings 22 (a source wiring or a drain wiring) connected to theimpurity region 13 are formed through contact holes (opening) providedin the first insulating film 16. In addition, a signal line, a powersupply line, and the like are formed in one layer in which the wiringsare formed. For example, the second insulating film 17 has an inorganicmaterial, which is formed to cover the wirings, the signal line, thepower supply line, and the like.

Through a contact hole provided in the second insulating film 17, thefirst electrode 19 is formed to connect to the wirings 22. Here, theauxiliary wiring 25 is formed in the layer of the first electrode 19.The auxiliary wiring 25 may be formed using the above material. Thethird insulating film 18 corresponding to a bank is formed to cover thefirst electrode 19 and the auxiliary wiring 25. For example, the thirdinsulating film 18 is formed to have an inorganic material. Thelight-emitting layer 20 is formed over the first electrode 19interposing a first contact hole provided in the third insulating film18.

When a full color display is obtained by coloring separatelylight-emitting layers of each color RGB, a light-emitting layer thatemits in white may be formed entirely. When using the light-emittinglayer of white light-emitting, a color filter and a color conversionlayer may be used for an opposite substrate side to obtain a full colordisplay. In addition, in carrying out a monochromatic display, an areacolor in which a light-emitting layer of predetermined color is formedmay be displayed.

Then, the second electrode 21 is formed to cover the light-emittinglayer 20. Here, a second contact hole is provided simultaneously in thethird insulating film 18 over the auxiliary wiring 25, in which thesecond electrode 21 and the auxiliary wiring 25 are connected throughthe second contact hole. The shape of the second contact hole can beformed to be in a line shape or a dot shape, or the combination thereof.

The first electrode 19 and the second electrode 21 can be an anode or acathode based on an emitting direction of light and polarity of thesemiconductor element. In this embodiment mode, the first electrode 19is taken as an anode and the second electrode 21 is taken as a cathode,which is described in the case where light is emitted to the secondelectrode side.

In this case, it is preferable to use a material with a large workfunction (at least work function 4.0 eV) such as metal, alloy, anelectrical conductive compound, and the compound thereof for an anodematerial. As a specific example of the anode material, gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), nitride(TiN), and the like of a metallic material can be used in addition toITO (indium tin oxide), IZO (indium zinc oxide) mixed zinc oxide (ZnO)of 2 to 20% into indium oxide.

On the other hand, it is preferable to use a material with a small workfunction (at most work function 3.8 eV) such as metal, alloy, anelectrical conductive compound, and the compound thereof for a cathodematerial. As a specific example of the cathode material, transitionmetal containing rare-earth metal can be used to form in addition to anelement belonging to Group. 1 or 2 element of the periodic table, thatis, alkali metal such as Li and Cs and alkaline earth metal such as Mg,Ca, and Sr; and alloy containing thereof (Mg: Ag, Al: Li) and thecompound (LiF, CsF, CaF₂). However, the cathode needs to be transparent;therefore, the metal or alloy containing the metal is formed to beextremely thin, which is formed to laminate with a transparentconductive film such as an ITO.

These anode and cathode can be formed by vapor deposition, sputtering,and the like.

A passivation film 29 comprising an insulating film mainly containingsilicon nitride or silicon nitride oxide that is obtained by sputtering(DC system and RF system), or a DLC film (Diamond Like Carbon)containing hydrogen is formed on the second electrode 21.

Accordingly, the pixel portion of the display device can be formed.

The auxiliary wiring 25 is formed over the second insulating film 17 anda configuration of the pixel portion of the display device in which afourth insulating film is provided is shown in FIG. 1B, which differsfrom FIG. 1A. Since the other configurations are same as FIG. 1A, itwill not be further explained.

The second insulating film 17 is formed so as to cover the wirings 22 toform contact holes. The auxiliary wiring 25 is provided over the secondinsulating film 17, and the auxiliary wiring 25 is formed in the contacthole. The third insulating film 18 is formed to cover the auxiliarywiring 25 to form contact holes. The first electrode 19 is formed overthe third insulating film and in the contact hole, so that the firstelectrode 19 is connected to the wiring 22 through the auxiliary wiring25. Furthermore, a fourth insulating film 26 corresponding to a bank isformed to cover the first electrode 19.

The light-emitting layer 20 is formed over the first electrode 19 andthe second electrode 21 is formed so as to cover the light-emittinglayer 20. Here, through the contact holes formed in the secondinsulating film 17 and the third insulating film 18, the secondelectrode 21 is connected to the auxiliary wiring 25.

A laminated constitution of the first to the third insulating films isnot limited to FIG. 1B, and furthermore, another insulating films may belaminated. The constitution for laminating the insulating films likeFIG. 1B is preferable since the constitution has less restriction on alayout for forming an electrode, a wiring, and the like. Especially,since there is little restriction on an area providing a light-emittinglayer, it is possible to enlarge the area of a light-emitting region.Furthermore, there is little restriction on an area for providing theauxiliary wiring; therefore, it is possible to form the auxiliary wiringin a much more enlarged area. As a result, it is possible to provide anelectrode and a wring with much more low resistance and to decreasepower consumption.

A configuration in which an inorganic material is included in aninsulating film is described above; however, an organic material can beused for an insulating film. An organic material has higher planaritycompared to an inorganic material. In addition, it can not necessary tocarry out etching for forming the contact hole if using appropriatematerial. Consequently, steps and dust can be reduced. There is aproblem of hygroscopicity for an organic material such as acryl andpolyimide; therefore, it is preferable to provide a protective film suchas a SiN film. Furthermore, resist has lower hygroscopicity compared toan organic material such as acryl and polyimide; therefore, it ispreferable that the use of a protective film such as SiN film can beeliminated, and moreover, takes lower cost compared to acryl andpolyimide, and it is preferable since a diameter of a contact holeformed by exposing to light gets shorter. However, resist often hascolor in most of the cases; therefore, a bottom emission type displaydevice in which light is emitted from a substrate side where asemiconductor element typified by a TFT is provided is suitable.

Thereafter, the case where an insulating film is formed using a resistas an organic material is described with reference to FIG. 1C. Otherconfigurations are the same as that of FIG. 1A, which will not befurther explained.

First, the wirings 22 so far is formed as in FIG. 1A, and simultaneouslythe auxiliary wiring 25 is formed. The auxiliary wiring 25 may be formedeither of a material same as that of the wirings 22 or a differentmaterial.

Thereafter in FIG. 1C, solution in which a cresol resin or the like ismelted in solvent (propylene glycol monomethyl ether acetate; PGMEA) isapplied as a positive type resist by spin coating. After the resist isapplied, the resist is heated at a temperature from 80° C. to. 150° C.using a heater (oven, hot plate) or the like and baked (referred to aspre-bake).

After the baking, a mask pattern to form a predetermined contact hole isdisposed in the second insulating film 17 and exposed. Then, the maskpattern is transferred to the resist. The positive type resist materialis used in this embodiment mode, so that an opening is provided at theposition emitted by light. Thereafter, when developer is dropped orsprayed, the position of the resist on which light is emitted melts andthe predetermined contact hole is formed in the second insulating film17. When a negative type material is used instead of the positive typematerial, an opening is provided at the position not emitted by light,and the position of the resist on which light is not emitted melts inthe developer and a contact hole is formed.

In forming an insulating film by using an organic material when thepredetermined thickness is not obtained, the solution may be appliedrepeatedly with each other, and the pre-bake and the application may becarried out over again.

After the contact hole is formed, heat treatment is carried out attemperatures from 120° C. to 250° C. using the heater (oven, hot plate)and the like to take off moisture and the like left within the resistand to stabilize much more (referred to as post-bake) simultaneously.

Among a plurality of contact holes formed in the second insulating film17, the first electrode 19 is formed in a first contact hole, which isconnected to the wirings 22. The auxiliary wiring 25 is exposed in asecond contact hole of the second insulating film 17. That is, thesecond contact hole is formed so that the side surfaces of the auxiliarywiring 25 does not contact with the edge portion of the secondinsulating film. The auxiliary wiring 25 may be formed after forming thesecond insulating film 17.

With the use of a resist material identical to that of the secondinsulating film 17 and a method thereof, the third insulating film 18corresponding to a bank is formed. A contact hole of the thirdinsulating film 18 is formed so that the auxiliary wiring 25 is exposedentirely. That is, a contact hole is formed so that the edge portion ofthe third insulating film 18 does not contact with the contact hole.

The light-emitting layer 20 is formed to cover the third insulating film18. Here, the light-emitting layer 20 is ended off and formed since afilm thickness of the light-emitting layer 20 is thin on the sidesurface of the auxiliary wiring 25. That is, the light-emitting layer 20is formed besides a part of the surface of the auxiliary wiring 25,specifically, besides a part of the side surface of the auxiliary wiring25.

The second electrode 21 is formed to cover the light-emitting layer 20.The second electrode 21 can have an electrical connection in order toform up to the side surface of the auxiliary wiring 25. For example, ametal film containing an element belonging to Group 1 or 2 element ofthe periodic table is formed to be thin. When the second electrode 21 isformed by laminating a transparent conductive film over the metal film,the metal film or the transparent conductive film may be electricallyconnected to the auxiliary wiring.

That is, in a configuration shown in FIG. 1C, it can be unnecessary toform a contact hole for electrically connecting the auxiliary wiring 25and the second electrode 21.

In the configuration shown in FIG. 1C, an organic material may befurther used for the first insulating film 16. It is preferable to forma plurality of insulating films with the same material since themanufacturing process becomes simple and easy.

Even in configurations shown in FIGS. 1A and 1B, it is possible to formthe light-emitting layer 20 to end off over the auxiliary wiring 25 whena contact hole is formed so that an insulating film cannot be providedat the edge portion of the auxiliary wiring 25 and the light-emittinglayer is formed over the entire surface of a pixel region.

As shown in FIGS. 1A to 1C, substantial resistance of the secondelectrode 21 can be reduced by providing the auxiliary wiring 25. As aresult, the reduction in the power consumption in the display device canbe obtained.

In addition, a signal writing defect or a gradation defect due to wiringresistance can be prevented. Furthermore, in the case of the secondelectrode 21, the generation of voltage drop can be controlled byconnecting with the auxiliary wiring 25, so that it becomes possible toapply same voltage to the light-emitting element. Consequently, theimprovement of the display quality can be obtained.

Especially in a large display device, an advantageous effect of reducingthe substantial resistance of an electrode and a wiring is remarkable.

Note that a layer for providing the auxiliary wiring is not limited tothe configuration shown in this embodiment mode. For example, theauxiliary wiring may be provided in one layer in which the gateelectrode is formed. Alternatively, a plurality of the auxiliary wiringsformed in a plurality of layers may be connected through the contactholes.

Not limiting to the configuration of a TFT shown in this embodimentmode, a configuration with a low concentration impurity region, aconfiguration in which an impurity region or a low concentrationimpurity region overlaps with a gate electrode, a configuration in whicha plurality of gate electrodes are provided for a semiconductor film, aconfiguration in which gate electrodes are provided to above and belowof a semiconductor film, and the like can be applied.

This embodiment mode can be applied to a top-emission type displaydevice in which light from a light-emitting layer is emitted to anopposed side of a substrate side where the semiconductor elementtypified by a TFT is provided, a bottom emission type display device inwhich light from a light-emitting layer is emitted to a substrate side,and dual emission type display device in which light emits to the bothsides.

(Embodiment Mode 2)

In this embodiment mode, an entire display device, especially, a leadwiring for connecting to an external circuit is described. Especially, alead wiring with the same potential as high-potential voltage VDD(hereinafter, described as an anode line) and a lead wiring with thesame potential as low-potential voltage VSS (hereinafter, described as acathode line) are described with reference to FIGS. 2A and 2B. In FIGS.2A and 2B, only a wiring disposed in a column direction in a pixelportion 104 is shown.

FIG. 2A is a top view of a panel, in which the pixel portion 104 where aplurality of pixels 105 are disposed in matrix a signal line drivercircuit 101, and scanning line driver circuits 102 and 103 around thepixel portion 104 are disposed on a substrate. The number of thesedriver circuits is not limited to FIG. 2A, and a plurality of signalline driver circuits or a single scanning line driver circuit may bedisposed according to a configuration of the pixels 105.

Signal lines 111 disposed in a column direction within the pixel portion104 are connected to the signal line driver circuit 101. Power supplylines 112 to 114 disposed in a column direction are each connected toany one of anode lines 107 to 109. Auxiliary wiring 110 disposed in acolumn direction is connected to a cathode line 106. The anode lines 107to 109 and the cathode line 106 are led so as to surround the drivercircuits disposed in the pixel portion 104 and the periphery, which isconnected to a terminal of an anisotropic film (FPC: Flexible PrintedCircuit) connecting to the external circuit.

It is preferable that the anode lines 107 to 109 are formedcorresponding to one of the colors of RGB. This is because the change ofeach of the potential of the anode lines 107 to 109 can correctvariation of a luminance generated between each color. That is, acurrent density of electroluminescent layers of light-emitting elementsdiffers in each color; therefore, the problem that a luminance becomesdifferent in each color even under the same current value can beresolved.

In this embodiment mode, it is assumed that case where a light-emittinglayer of RGB is colored separately. However, as a method ofcolorization, when a method in which the difference of a current densityin each color is not problematic, for example, when a method for using alight-emitting layer that emits white and a color filter is adopted, itis not necessary to provide a plurality of anode lines.

FIG. 2B is a mask layout diagram, in which the anode lines 107 to 109and the cathode line 106 are disposed around the signal line drivercircuit 101, and the anode lines 107 to 109 are connected with the powersupply lines 112 to 114 disposed in a column direction in the pixelportion 104 through a contact hole.

In this embodiment mode, the cathode line 106, the anode lines 107 to109 are formed of a conductive film of one layer in which the auxiliarywirings 110 is formed. The auxiliary wiring 110 is formed of a materialwith lower resistance; therefore, it is preferable to assume the cathodeline 106 and the anode lines 107–109 that are led so as to surround thedriver circuit as a conductive film of one layer.

After forming the cathode line 106, the anode lines 107 to 109, and theauxiliary wirings 110, the first electrode of a light-emitting elementis formed, and an insulating film corresponding to a bank is formed. Acontact hole is formed in the insulating film which is placed over aregion in which the cathode line 106 is formed, a region forming alight-emitting layer, and a region in which the auxiliary wiring isformed. The cathode 106, the first electrode, and the auxiliary wirings110 are exposed by forming the contact hole. As shown in FIGS. 1A to 1C,a light-emitting layer is formed over the contact hole on the firstelectrode. Here, a light-emitting layer of each RGB may be coloredseparately by a metal mask to evaporate, and a white light-emittinglayer may be evaporated to the entire surface.

Next, a second electrode that covers the light emitting layer is formed.Here, the second electrode formed on the light-emitting layer isconnected not only to the cathode lines 106 but also to the auxiliarywirings 110 disposed in a column direction within the pixel portion 104.Due to the configuration in which the second electrode and the auxiliarywiring 110 are connected in the pixel portion, the substantialresistance of the second electrode can be reduced. Therefore, theproblem of a defect in image quality and a high power consumption due toresistance of the second electrode can be improved.

A layer for forming the auxiliary wirings 110 is not limited to aconductive film of which layer is the same as the signal lines as shownin FIG. 2B, and a conductive film of one layer in which the scanninglines are formed may be used. In addition, a shape of the contact holebetween the auxiliary wirings 110 and the second electrode is notlimited to FIG. 2B, and it may be provided in a linear shape or in aspotted shape in a column direction. Hereinafter, a layout of thecontact holes between the auxiliary wirings 110 and the second electrodeis described with reference to FIGS. 3 to 6 by giving some examples.Note that the signal lines 111, the auxiliary wirings 110, the cathodeline 106, and scanning lines 120 in a pixel portion 104 are shown inFIGS. 3 to 6.

In FIG. 3, a contact region 121 of the auxiliary wirings 110 and thecathode line 106 is formed. The auxiliary wirings 110 and the secondelectrode in each pixel 105 are connected through contact holes 122 in around shape (dot shape).

In FIG. 4, the contact region 121 of the auxiliary wirings 110 and thecathode line 106 is formed. The auxiliary wirings 110 and the secondelectrode in each pixel 105 are connected through contact holes 122 in alinear shape (line shape). In other words, the contact region 121 andthe contact holes 122 in a linear shape are formed simultaneously andconnected.

The contact holes 122 shown in FIG. 4 are formed to be larger than theauxiliary wirings 110. In the case of the configuration shown in FIG.1C, the contact holes 122 are bigger than the auxiliary wirings 110. Inthe case of the configuration shown in FIGS. 1A and 1B, the contactholes 122 are smaller than the auxiliary wirings 110.

In FIG. 5, the contact region 121 of the auxiliary wirings 110 and thecathode line 106 is formed. The two auxiliary wirings 110 are providedin each pixel 105, in which these auxiliary wirings 110 and the secondelectrode are connected through the contact holes 122 in a round shape(dot shape). The contact holes 122 in a round shape are formed at fourcorners of each pixel.

Accordingly, a plurality of auxiliary wirings 110 may be provided in onepixel. Furthermore, the plurality of auxiliary wirings 110 may beprovided by laminating them.

In FIG. 6, the auxiliary wirings 110 are formed in one layer in whichthe gate wiring is formed. The contact region 121 of the auxiliarywirings 110 and the cathode line 106 is formed, in which the auxiliarywirings 110 and the second electrode in each pixel 105 are connectedthrough a shape with some area (area shape).

As shown in FIGS. 3 to 6, there can be various layouts of the contactholes 122 with the auxiliary wirings 110 and the second electrode. Ashape of the contact holes such as a round shape, a linear shape, or anarea shape can be combined with any one of the configurations shown inFIGS. 3 to 6.

When the connection between the auxiliary wirings 110 and the secondelectrode is made enough through the contact holes 122 in the pixelportion, the contact region 121 with the cathode line 106, and thecathode line 106 below the contact region 121 can be unnecessary. Inthis case, it is preferable to use one layer in which the auxiliarywiring, the gate wiring, or source and drain wirings is formed for thelead wiring for connecting the second electrode to the FPC.

(Embodiment Mode 3)

In this embodiment mode, an equivalent circuit of a pixel portion of adisplay device is described.

A pixel circuit shown in FIG. 7A comprises a light-emitting element 39,a signal line 30 in which a video signal is input, a transistor(switching transistor) 35 used for a switching element for controllingthe input of the video signal into a pixel, a transistor (drivetransistor) 36 for controlling current value flown into thelight-emitting element 39, a transistor (current control transistor) 37for controlling the supply of current to the light-emitting element 39,and an auxiliary wiring 34 connected with a second electrode of thelight-emitting element 39. Furthermore, a capacitor element 38 forholding the potential of the video signal may be provided.

The drive transistor 36 and the current control transistor 37 are formedso as to have a same conductivity type. This embodiment mode describesthe case of a p-channel type.

In this embodiment mode, the drive transistor 36 is operated in asaturation region, and the current control transistor 37 is operated ina linear region. Therefore, the L(channel length) of the drivetransistor 36 may be longer than the W(channel width), and the L(channellength) of the current control transistor 37 may be the same or shorterthan the W W(channel width). More preferably, the ratio of the drivetransistor 36 of the W(channel width) to the L(channel length) may be nofewer than 5.

An enhancement mode transistor may be used or a depletion modetransistor may be used for the drive transistor. This embodiment mode isdescribed in the case where a depletion type transistor is used.

A gate electrode of the switching transistor 35 is connected to ascanning line 31. As for a source region and a drain region of theswitching transistor 35, one is connected to the signal line 30 and theother is connected to a gate electrode of the current control transistor37. A gate electrode of the drive transistor 36 is connected to a secondpower supply line 33. The drive transistor 36 and the current controltransistor 37 are connected to a first power supply line 32 and thelight-emitting element 39, so that a current supplied by the first powersupply line 32 is supplied to the light-emitting element 39 as draincurrents of the drive transistor 36 and the current control transistor37. In this embodiment mode, a source region of the current controltransistor 37 is connected to the first power supply line 32, and thedrain region of the drive transistor 36 is connected to a firstelectrode of the light-emitting element 39.

Note that a source region of the drive transistor 36 is connected to thefirst power supply line 32, and the drain region of the current controltransistor 37 may be connected to the first electrode of thelight-emitting element 39.

Potential difference is given to the second electrode and the firstpower supply line 32 respectively so that current of forward biasdirection is provided to the light-emitting element 39.

Furthermore, the second electrode is connected to the auxiliary wiring34, which reduces the substantial resistance of the second electrode. Itis preferable to form the auxiliary wiring 34 using a conductive film ofone layer in which the signal line 30, the first power supply line 32,and the second power supply line 33 are formed, and the auxiliary wiring34 may be formed in one layer in which the first electrode is formed asshown in FIG. 1A.

One of two electrodes comprised in a capacitor element 38 is connectedto the first power supply line 32, and the other is connected to thegate electrode of the current control transistor 37. When the switchingtransistor 35 is in a non-selected state (OFF state), the capacitorelement 38 is provided to keep potential difference between electrodesof the capacitor element 38. However, when the leak current from eachtransistor is small, the gate capacitance of the switching transistor35, the drive transistor 36, or the current control transistor 37 islarge, it is not necessary to provide the capacitor element 38.

The drive transistor 36 and the current control transistor 37 arep-channel type transistors, in which the source region of the drivetransistor 36 and an anode of the light-emitting element 39 areconnected in FIGS. 1A to 1C. Conversely, when the drive transistor 36and the current control transistor 37 are n-channel type transistors,the source region of the drive transistor 36 and a cathode of thelight-emitting element 39 are connected.

Next, a driving method of a pixel shown in FIG. 7A is described bydividing into a writing period and storage time. First, when thescanning line 31 is selected in the writing period, the switchingtransistor 35 connected to the scanning line 31 is turned ON. Then, thevideo signal input to the signal line 30 is input to the gate electrodeof the current control transistor 37 through the switching transistor35. The drive transistor 36 is connected to the first power supply line32; therefore, it is always turned ON.

When the current control transistor 37 is turned ON by a video signal, acurrent is flown through the light-emitting element 39 through the firstpower supply line 32. Here, since the current control transistor 37 isoperated in a linear region, the current flown through thelight-emitting element 39 depends on the drive transistor 36 operated ina saturation region and a current-voltage characteristic of thelight-emitting element 39. The light-emitting element 39 emits light ina luminance corresponding to the current that is provided.

In addition, when the current control transistor 37 is turned OFF by avideo signal, the light-emitting element 39 is not supplied with acurrent.

In the storage time, the switching transistor 35 is turned OFF bycontrolling the potential of the scanning line 31, in which thepotential of the video signal written in the writing period is held.When the current control transistor 37 is turned ON in the writingperiod, the potential of the video signal is held by the capacitorelement 38; therefore, the light-emitting element 39 is continued to besupplied with a current. On the contrary, when the current controltransistor 37 is turned OFF in the writing period, the potential of thevideo signal is held by the capacitor element 38; therefore, thelight-emitting element 39 is not supplied with a current.

A pixel circuit shown in FIG. 7B is different from that shown in FIG. 7Ain a configuration in which a transistor (erase transistor) 40 isprovided to erase the potential of the written video signal. A gateelectrode of the erase transistor 40 is connected to a second scanningline 41, as for a source and a drain, one is connected to the firstpower supply line 32 and the other is connected to the gate electrode ofthe current control transistor 37.

Other configurations are the same as that shown in FIG. 7A, and thesecond electrode of the light-emitting element 39 is connected to theauxiliary wiring 34, which reduces the substantial resistance of thesecond electrode.

Next, a driving method of a pixel shown in FIG. 7B can be described byseparating into an erase period in addition to a writing period and astorage time.

In the erase period, the second scanning line 41 is selected to turn theerase transistor 40 ON, in which the potential of the power supply line32 is given to the gate electrode of the current control transistor 37through the erase transistor 40. Accordingly, the current controltransistor 37 is turned OFF; therefore, a state in which thelight-emitting element 39 is forced not to supply with a current can bemade.

A pixel circuit shown in FIG. 7C is different from that of FIG. 7A in aconfiguration in which the gate electrode of the drive transistor 36 isconnected to a third scanning line, 45. The gate electrode of the drivetransistor 36 may be connected to a wiring provided with a constantpotential. It is preferable to form the auxiliary wiring 34 by using aconductive film of one layer in which the first scanning line 31 and thethird scanning line 45 are formed.

Other configurations are the same as that of FIG. 7A, and the secondelectrode of the light-emitting element 39 is connected to the auxiliarywiring 34, which reduces the substantial resistance of the secondelectrode.

A driving method of a pixel shown in FIG. 7C is the same as the drivingmethod described referring to FIG. 7A, which will not be furtherexplained.

Similar to FIG. 7B, a pixel circuit shown in FIG. 7D has a configurationin which the erase transistor 40 is provided for the pixel circuit shownin FIG. 7B.

Other configurations are the same as that of FIG. 7C, and the secondelectrode of the light-emitting element 39 is connected to the auxiliarywiring 34, which reduces the substantial resistance of the secondelectrode.

The driving method of a pixel shown in FIG. 7C is the same as thedriving method described referring to FIG. 7B, which will not be furtherexplained.

A pixel circuit shown in FIG. 7E is different from that of FIG. 7B in aconfiguration in which the driving transistor 36 is not provided.

It is preferable to operate the current control transistor 37 in asaturation region so that the drive transistor 36 is not affected by thedegradation of the light-emitting element. In operating the drivetransistor 36 in a saturation region, it is necessary to considervoltage including a margin of voltage drop due to the second electrodeand a margin of the degradation of the light-emitting element. However,the margin of the voltage drop due to the second electrode can be madeunnecessary by the auxiliary wiring, which can result in a low powerconsumption of the display device.

Other configurations are the same as that of FIG. 7A, and the secondelectrode of the light-emitting element 39 is connected to the auxiliarywiring 34, which reduces the substantial resistance of the secondelectrode.

A driving method of a pixel shown in FIG. 7E is the same as the drivingmethod described referring to FIG. 7B, which will not be furtherexplained.

Similar to FIGS. 7A and 7C, it is needless to say that an erasetransistor may not be provided in the pixel circuit in FIG. 7E.

Although the case of the pixel type in which the voltage signal is inputas the video signal into the signal line 30 is described in FIGS. 7A to7E, a pixel type in which a current signal is input as the video signalinto the signal line 30 may be used. Since the substantial resistance ofa wiring and an electrode can be reduced, the voltage drop due to thehigh resistance can be prevented. Therefore, the configuration havingthe auxiliary wiring applied in accordance with the pixel type in whichthe voltage signal is input results in a prominent advantageous effect.

In addition, the pixel circuit having the light-emitting element isdescribed; however, a configuration including the auxiliary wiring in apixel circuit having a liquid crystal element may be used.

(Embodiment Mode 4)

In this embodiment mode, an example of a top view of a pixel portioncorresponding to the equivalent circuit shown in FIG. 7B is described.

FIG. 8 comprises a signal line 801, a first power supply line 802, asecond scanning line 803, a first scanning line 804, a switchingtransistor 805, an erase transistor 806, a drive transistor 807, acurrent control transistor 808, a first electrode 809, an auxiliarywiring 810, a second power supply line 811, and a capacitor element 812.

In this embodiment mode, the signal line 801, the first power supplyline 802, and the second power supply line 811 are formed by patterningthe same conductive film as signal line 801 and so on. In addition, asource wiring and a drain wiring of a transistor are formed of the sameconductive film. The first scanning line 804 and the second scanningline 803 are formed by patterning the same conductive film. Furthermore,a part of the first scanning line 804 and the second scanning line 803are overlapped with a portion of semiconductor film, and being operatingas a gate electrode.

The auxiliary wiring 810 is formed by interposing an insulating filmover the first power supply line 802 and the second power supply line811. Therefore, the auxiliary wiring 810 in a large area can be formed.When capacitance is generated between the auxiliary wiring and the firstpower supply line, and the auxiliary wiring and the second power supplyline, a part of the auxiliary wiring may be used as a capacitor element.In addition, an unnecessary capacitance can be decreased by using aLow-K material for an insulating film. It is also possible to form theauxiliary wiring 810 in one layer in which the first power supply line802 and the second power supply line 811 are formed. In this case, afilm thickness of the auxiliary wiring is decided in order to obtainpredetermined resistance.

In order to operate the drive transistor 807 in a saturation region, itis designed so that L(channel length)/W(channel width) being bigger thanthat of the current control transistor 808. For example, it is set that(L(channel length)/W(channel width) of the driving transistor):(L(channel length)/W(channel width) of the current controltransistor)=(5 to 6000):(1). Therefore, a semiconductor film of thedrive transistor 807 is formed in a rectangular.

The capacitor element 812 comprises a protective film containing SiNsandwiched between the second power supply line 811 and thesemiconductor film of the drive transistor 807, and a second insulatingfilm.

Next, FIGS. 9A to 9C show a cross-sectional views of devices in whichthe auxiliary wiring 810 is formed.

FIG. 9A corresponds to a cross-section of A–A′ in FIG. 8, which showsthe cross-sectional view of the switching transistor 805 and the erasetransistor 806, and the auxiliary wiring 810 formed over the erasetransistor 806.

FIG. 9B corresponds to a cross-section of B–B′ in FIG. 8, which shows across-sectional view of the drive transistor 807; the capacitor element812 formed by sandwiching the second power supply line 811 and thesemiconductor film of the drive transistor 807; a part of asemiconductor film of the current control transistor 808; the firstelectrode 809; and the auxiliary wiring 810. The drive transistor 807and the current control transistor 808 may have a LDD (Lightly DopedDrain) structure with a low concentration impurity region or a GOLD(Gate-drain Overlapped LDD) structure in which a low concentrationimpurity region overlapped by a gate electrode.

FIG. 9C corresponds to a cross-section of C–C′ in FIG. 8, which shows across-sectional view of the second power supply line 811, the firstelectrode 809, and the auxiliary wiring 810.

FIGS. 10A to 10C show cross-sectional views in which a third insulatingfilm corresponding to a bank is formed on the auxiliary wiring 810, alight-emitting layer 815 is formed in an opening of the third insulatingfilm, and a second electrode 816 is formed covering the light-emittinglayer 815.

FIGS. 10A and 10C correspond to a cross-section of A–A′ and C–C′ in FIG.8, each of which shows a cross-sectional view of the case where thethird insulating film is formed over the auxiliary wiring 810. Inaddition, FIG. 10B corresponds to a cross-section of B–B′ in FIG. 8,which shows a cross-sectional view in which a first contact hole and asecond contact hole are formed in the third insulating film over thefirst electrode 809 and the auxiliary wiring 810, the light-emittinglayer 815 is formed in the first contact hole, and a second electrode isformed in the second contact hole covering the light-emitting layer 815.

Configurations shown in FIGS. 8A to 10C correspond to the configurationshown in FIG. 1A; however, also the configurations shown in FIGS. 1B and1C can be used in this embodiment mode.

Thus, the second electrode 816 and the auxiliary wiring 810 areconnected, which can reduces the substantial resistance. Consequently,reduction in the power consumption of the display device can beachieved.

In addition, a signal writing defect, a gradation defect, and the likedue to a wiring resistance can be prevented. Furthermore, in the case ofthe second electrode, voltage drop can be supressed by being connectedto the auxiliary wiring, so that it becomes possible to apply samevoltage to light-emitting elements. Consequently, the improvement of thedisplay quality can be obtained.

Especially in a large display device, an advantageous effect of reducingthe substantial resistance of an electrode and a wiring is remarkable.

(Embodiment Mode 5)

A display device and an electronic device of the present inventioninclude a video camera, a digital camera, a goggle type display (headmounted display), a navigation system, an audio reproducing device (acar audio, an audio component, and the like), a laptop computer, a gamemachine, a portable information terminal (a mobile computer, a cellularphone, a portable game machine, an electronic book, or the like), animage reproducing device (specifically a device capable of producing arecording medium such as a Digital Versatile Disc (DVD) and having adisplay device that can display the image) and the like. Especially, itis preferable to use the auxiliary wiring of the invention for alarge-sized television with a large-sized screen and the like. Specificexamples of the electronic devices are shown in FIGS. 13A to 13C.

FIG. 13A is a large-sized display device, which includes a chassis 2001,a support 2002, a display portion 2003, a speaker portion 2004, and avideo input terminal 2005. The auxiliary wiring of the invention isconnected to a wiring and an electrode provided for the display portion2003, which can reduce the substantial resistance of the wiring and theelectrode. As a result, voltage drop and depression of a signal can bereduced in a large-sized display device with a long wiring length. Thedisplay device includes every display devices for displaying informationfor a personal computer, for a TV broadcast reception, for anadvertisement display, and the like.

FIG. 13B is a laptop computer, which includes a main body 2201, achassis 2202, a display portion 2203, a keyboard 2204, an externalconnection port 2205, a pointing mouse 2206, and the like. The auxiliarywiring of the invention is connected to a wiring and an electrodeprovided for the display portion 2203, which can reduce the substantialresistance of the wiring and the electrode.

FIG. 13C is a portable image reproduction device equipped with arecording medium (specifically, a DVD player), which includes a mainbody 2401, a chassis 2402, a display portion A 2403, a display portion B2404, a recording medium (a DVD players and the like) reading portion2405, operation keys 2406, speaker portions 2407, and the like. Thedisplay portion A 2403 mainly displays image information whereas thedisplay portion B 2404 mainly displays text information. The auxiliarywiring of the invention is connected to wirings and electrodes providedfor these display portions A 2403 and B 2404, which can reduce thesubstantial resistance of the wirings and the electrodes. The imagereproduction device equipped with a recording medium includes home videogame machines and the like.

As described above, the application range of the invention is extremelywide; therefore, the invention can be applied to the electronic devicesof every field. In addition, the electronic devices shown in thisembodiment mode can use any one of configurations shown in EmbodimentMode 1 to 4.

EMBODIMENT

In this embodiment, Al—Si and Al—Ti are used for a material of anauxiliary wiring. When the line width of the auxiliary wiring of onelength is changed in the range of 2 μm to 82 μm, a film thicknessnecessary for obtaining resistance value of 0.01 Ω, 0.1 Ω, 1 Ω, and 5 Ωis calculated. The result used Al—Si is shown in FIG. 11 and the resultused Al—Ti is shown in FIG. 12.

Using the computation expression: R=R_(real)×(d_(s)/d)×(W_(s)/W), Rrepresents resistance value that can be obtained by changing the widthof the auxiliary wiring and the film thickness, where W: a real width ofthe auxiliary wiring, d: a real film thickness of the auxiliary wiring,W_(s): a width of the auxiliary wiring in designing, d_(s): a filmthickness of the auxiliary wiring in designing, and R_(real): a realresistivity in each material. Real resistivity of Al—Si and Al—Ti:R_(real) is 4.1×10⁻⁶ Ω·cm, 8.5×10⁻⁶ Ω·cm, respectively.

Desired resistance value of the auxiliary wiring is changed due to thepanel size of the display device. The larger the panel size becomes, thelower resistance value of the auxiliary wiring is required since thewiring becomes long. Here, the resistance value of 0.1 Ω is discussed.According to FIGS. 11 and 12, it can be understood that the auxiliarywiring needs to have the width of about 30 μm and the film thickness of4000 Å (400 nm) when Al—Si is used, and the width of about. 60 μm andthe film thickness of 4000 Å (400 nm) when Al—Ti is used in order toobtain the resistance value of 0.1 Ω.

Although there is a limitation on the width and the film thickness ofthe auxiliary wiring, the film thickness of 4000 Å (400 nm) is a valuethat can be realized. In addition, in the case of the bottom emissiontype display device, it is not desirable that the width of the auxiliarywiring exceeds the insulating film corresponding to a bank, consideringthe aperture ratio. Therefore, when the auxiliary wiring is required tohave a width of the bank, the auxiliary wiring may be laminated.

Furthermore, when the auxiliary wiring is formed as a top emission typedisplay device in a layer different from that of an anode as shown inFIG. 1B, the limit of the width of the auxiliary wiring is not requiredto have the width of the bank. Consequently, much lower sheet resistancecan be obtained.

Substantial resistance can be reduced by connecting the auxiliary wiringto the electrode or the wiring of the display device. Consequently,reduction in the power consumption of the display device can beachieved.

In addition, a signal writing defect, a gradation defect, and the likedue to a wiring resistance can be prevented. Furthermore, the generationof voltage drop can be controlled so that it becomes possible to applyuniform amount of voltage to a light-emitting element. Consequently, theimprovement of the display quality can be obtained.

Especially in a large display device, an advantageous effect of reducingthe substantial resistance of an electrode and a wiring is remarkable.

1. A display device comprising: a first insulating film over asemiconductor film; a conductive film comprising a first material on thefirst insulating film; a first electrode comprising a second material onthe first insulating film; a second insulating film over the firstinsulating film, the conductive film, and the first electrode; alight-emitting layer over an opening of the second insulating film andthe first electrode; and a second electrode over the light-emittinglayer and the second insulating film, wherein the second electrode isconnected to the conductive film; and wherein the first material isdifferent from the second material.
 2. The display device according toclaim 1, wherein the second electrode is transparent.
 3. The displaydevice according to claim 1, wherein the semiconductor film is acrystalline thin-film semiconductor.
 4. The display device according toclaim 1, wherein the first insulating film contains an organic materialor an inorganic material.
 5. The display device according to claim 1,wherein the second insulating film contains an organic material or aninorganic material.
 6. The display device according to claim 1 whereinthe display device further comprises a signal line, and the conductivefilm is provided in one layer in which the signal line is formed.
 7. Thedisplay device according to claim 1, wherein the display device furthercomprises a signal line, and the conductive film contains one materialwith which the signal line is formed.
 8. The display device according toclaim 1, wherein the display device further comprises a scanning line,and the conductive film is provided in one layer in which the scanningline is formed.
 9. The display device according to claim 1, wherein thedisplay device further comprises a scanning line, and the conductivefilm contains one material with which the scanning line is formed. 10.The display device according to claim 1, wherein the conductive filmconnects an external circuit.
 11. The display device according to claim1, wherein the display device is one selected from the group consistingof a video camera, a digital camera, a goggle type display, a navigationsystem, an audio reproducing device, a laptop computer, a game machine,a portable information terminal and an image reproducing device.
 12. Thedisplay device according to claim 1, wherein a resistance of the firstmaterial is lower than a resistance of the second material.
 13. Adisplay device comprising: a thin film transistor; a first insulatingfilm formed over the thin film transistor; an auxiliary wiringcomprising a first material formed on the first insulating film; a firstelectrode comprising a second material formed on the first insulatingfilm and electrically connected to the thin film transistor, a secondinsulating film having a first contact hole and second contact hole onthe auxiliary wiring and the first electrode; a light-emitting layerover the first electrode at least in the first contact hole; and asecond electrode over the light emitting layer; wherein the secondelectrode is connected to the auxiliary wiring in the second contacthole, and wherein the first material is different from the secondmaterial.
 14. The display device according to claim 13, wherein thedisplay device is one selected from the group consisting of a videocamera, a digital camera, a goggle type display, a navigation system, anaudio reproducing device, a laptop computer, a game machine, a portableinformation terminal and an image reproducing device.
 15. The displaydevice according to claim 13, wherein a resistance of the first materialis lower than a resistance of the second material.